This application claims the benefit of a Japanese Patent Application No. 2001-045385 filed Feb. 21, 2001, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to phase compensation methods and optical storage apparatuses, and more particularly to a phase compensation method for compensating an optical phase to improve a quality of a signal reproduced from an optical recording medium, and to an optical storage apparatus which employs such a phase compensation method.
As one example of an optical storage apparatus, there is an optical disk unit which writes information on and reads information from a disk by converging light emitted from a light source on the disk via a condenser lens. The disks used by such an optical disk unit include read-only CD-ROMs, DVD-ROMs, writable phase change type optical disks, magneto-optical disks and the like.
2. Description of the Related Art
In the case of a magneto-optical disk unit, information is reproduced from the magneto-optical disk by detecting a rotation direction of a polarization plane which is generated by magnetizations which are recorded in a perpendicular direction to a recording surface of the magneto-optical disk. As methods of increasing the recording density of the magneto-optical disk, there is a method which increases the recording density in a circumferential direction of the magneto-optical disk, and a method of increasing the recording density in a radial direction of the magneto-optical disk. In order to increase the recording density in the radial direction of the magneto-optical disk, a so-called land-groove recording system has been proposed in addition to a land recording system which records the information only on lands of the recording surface of the magneto-optical disk. The land-groove recording system records the information on both the lands and grooves of the recording surface of the magneto-optical disk. The groove is formed between two adjacent lands of the recording surface. When the land-groove recording system is employed, it is possible to reduce the apparent track pitch.
When employing the land recording system, an optimum value of an optical phase of a reproducing optical system of the magneto-optical disk unit is close to 0 degree, and a phase plate for phase compensation is provided to adjust phases of optical parts and a phase of the optical system as a whole. On the other hand, when employing the land-groove recording system, a depth of the groove with respect to the land is made greater than a predetermined value, so that a high-quality signal can be reproduced by suppressing undesirable effects of crosstalk from the adjacent tracks. For this reason, an optimum amount of phase compensation differs between a case where the signal is reproduced from the land and a case where the signal is reproduced from the groove. Therefore, when employing the land-groove recording system, it is necessary to switch the amount of phase compensation between the land signal reproduction and the groove signal reproduction, by switching an inclination angle of the phase plate.
However, in the case of the magneto-optical disk unit which is designed for the magneto-optical disk employing the land-groove recording system, the structure of the magneto-optical disk unit makes it impossible to reproduce the signal when the amount of the phase compensation is close to 0 degree. Consequently, there was a first problem in that this magneto-optical disk unit is unsuited for reproducing signals from a magneto-optical disk employing the land recording system.
On the other hand, even among the magneto-optical disks employing the same land-groove recording system, the optimum amount of phase compensation differs depending on the specifications such as the wavelength of the light from the light source which is used, the depth of the groove and the track pitch. For this reason, in the case of a magneto-optical disk unit which is designed for a magneto-optical disk having a predetermined specification and employing the land-groove recording system, there was a second problem in that such a magneto-optical disk unit is unsuited for reproducing signals from magneto-optical disks having different specifications.
Furthermore, in the magneto-optical disk employing either the land recording system or the land-groove recording system, the optimum amount of phase compensation may deviate even within the same track (one track turn), the same zone or among a plurality of tracks of the magneto-optical disk due to inconsistencies introduced during the production process of the magneto-optical disk and inconsistent mounting positions of parts of the magneto-optical disk unit. But when the amount of phase compensation is switched between two fixed values between the land signal reproduction and the groove signal reproduction, there was a third problem in that it is impossible to cope with the deviation in the optimum amount of phase compensation that occurs within each track.